Thermal current-responsive device



Jan. 17, 1939. T, A, RI H 2,144,589

THERMAL CURRENT -RE |SPONS IVE DEVICE Filed July 14, 1934 Z 9 25 6 Lu 2? LI LU 2/ D lrwventor: TheodoreAPich,

ttorney Patented Jan. 17, 1939 THERMAL CURRENT-RESPONSIVE DEVICE Theodore A. Rich, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York i Application July 14, 1934, Serial No. 735,227 15 Claims. (oi. 171-95) construction for current-responsive devices which fundamental form of apparatus embodying my 5 permitsobtaining the greater sturdiness and durinvention; Fig. 2 represents the modification of ability, the high available torque and currentthe fundamental form in which the bimetallic carrying capacity, and the relatively low manustrip is coiled for the sake of greater compactfacturing cost of devices employing bimetallicness; Fig. 3 represents another embodiment of strip for the torque-producing element without my invention; Fig. 4 represents still another em- 0 obtaining the slowness of response ordinarily inbodiment of my invention; and Fig. 5 is a graph herent in such devices. showing the relationship between the deflection Other and further objects and advantages will of the bimetallic strips and time, and explaining become apparent as the description proceeds. the principle of operation of my invention.

l5 Owing to the high torque and current-carrying Referring now more in detail to the drawing in capacity of bimetallic'stripcurrent-indicating inwhich .like reference characters are utilized to struments,,it is ordinarily not desirable to emdesignate like parts throughout, in Figs. lto 4,

ploy flexible leads in connection with such instruthe torque-producing element of my current.- ments. Consequently, U-shaped members formed responsive device comprises an E-shaped sheet of of bimetallic strips are frequently emplbyed to bimetal ll. By. the tellll E-shaped as used in the 20 permit, t ends of t arms t th open d of description and claims, I mean formed from a the U to be permanently-fastened to the stationeet t e de p e t of which is a figure 6 ary element and to permit making the current sisting of th ee arms, free at one end and. jo ned connections to such ends. The movable pointer y a yoke at the other end to simulate an E or or movable contact, as the case may be, is then wo Us moved t et er, as illustrated in attached to the yoke portion of the U and, of Although p te strips of etal may be em course, deflects as variations in current flowing ployed, I find it convenient and satisfactory to through the bimetallic strips cause variations in amp the t reee ged p figures temperature thereof and tend to cause the strip the torque-producing elements directly from a to warp or twist in one direction or the-other. I sheet of bimetal. 30

In accordance with my invention in its pre- The bimetal to which I refer is a well known ferred form, I also make use of a similar construcarticle of commerce and consists of two sheets or tion but I add an additional arm to form an E- strips of materials having different temperature shaped member instead of a, simple U-shaped coefficients of expansion laid together. and united member and the additionalarm carries no curalong their common surfaces in any suitable manrent but is, of course, influenced by variations in Her as by welding. For example, brass d S e temperature of the other two arms. The free end re o ly p yed and, in that case, since of the non-current carrying arm serves as the brass has the greater temperature coefficient of support for movable contacts or the deflecting expansion. as the te p tu e ses, the bimetal 40 pointer. Variations in temperature tend to dew ld bend away from th i compo f brass 40 fleet the end of the current-free arm in a direcand, as the temperature decreases, it would bend tion opposite that which will be caused by the way fr m he si mp f l. twisting of the current-carrying arms. Consea y. the y Portion ll of t e bimetal quently, the-current-free arm serves to cofnpene ement II is strengthened andalsopreventedfrom sate for variations in ambient temperature. be ding by channeling or crimping as illustrated. 4

I diti t thi ff ct, h owing to For a reason which will be explained hereinafter, the gradual heating of the current-free arm from .I find that it may a so be desirable to Crimp the radiation produced by the current-carrying channel the end portions l3 of the current-carryarms, the gradual creepage in deflection of the ng arms of the bimetal member II. The chancurrent-carrying arms, resulting from the fact neled ends l3 are fastened to current terminals 5!? that the bimetallic strip is not heated instantanel4 rigidly supported in the stationary portion of ously by the current, is largely overcome by the the apparatus, not shown, and current is led to opposite creepage of the current-free arms so the device through conductors 45. The free or that the pointer reaches its ultimate indicating non-current carrying arm It may carry a conposition much more quickly than would be the tact or, as shown, an indicating pointer ll 00- 55 case in ordinary bimetallic strip indicating inoperating with a suitable scale [8. struments. In'the arrangement of Fig. 2, the bimetallic ele- The features of my invention which I believe to ment l I is coiled up to form three spirals after it be novel and patentable will be pointed out in the has been stamped from the sheet of bimetal but it will be understood that there is no difference in My invention relates to current-responsive devices and' concerns particularly devices of the thermal type employing a bimetallic strip.

It is an object of my invention to provide a I claims appended hereto. A better understanding of my invention, itself, however, may be obtained from the following description taken in connection with the accompanying drawing in which Fig. 1 represents schematically .in perspective a the principle 01' operation since a given bend per unit length of the bimetallic strips results in the same angle of twist at the unsupported end regardless of the original shape of the strip, unless the radius of curvature is very small, smaller than is used here in the active portions of the bimetal.

As the current is led through conductors l5, arms is, and yoke ii, the unsupported ends of the arms I! and the yoke portion i2 are obviously deflected in one direction or the other by reason of the heating oi arms is caused by the current. The temperature, however, ,rises exponentially with time so that the deflection of the unsupported ends of the strips I! also varies exponentially with time, as shown in curve 20 of Fig. 5. The ultimate position of the pointer would, therefore, not be attained for a considerable period after current has started to flow in the arms is if the indicating pointer were connected to the yoke l2. The actual ultimate position would correspond to that at which heat dissipation from arms I! equaled the heat energy supplied by the current flowing in arms i9, which would theoretically occur after an infinite period of time. However, after a certain finite period of time, further change in position would become inappreciable. In practice, it has been found that only after six or seven minutes does no further appreciable deflection occur.

The arm It being situated between the current-carrying arms I9 obviously is gradually heated by heat given oii by arms is and, since it is supported at the opposite end with respect to the current-carrying arms iii, the deflection oi the pointer l1 due to heating of arm l6 tends to be in a direction opposite that produced by the heating of the arms IS. The deflection of pointer i1 due to the heating of arm [6 alone is represented by curve 2| of Fig. 5. It will be seen that this component of the deflection also increases exponentially and does not attain its apparent ultimate value until a lapse of a considerable length 0! time, for example, six or seven minutes. The actual deflection of the pointer l1, however, is the diflerence between the components of deflection 10 and deflection 2| produced by the heating of arms l9 and i8, respectively. The latter portions of curves 20 and II in which change 0! deflection is relatively gradual tend substantially to cancel each other so that, after a relatively short period of time, for example, after about one and one-half minutes in one actual instrument, at the instant represented by the point 22, the pointer I! ceases to be appreciably further deflected and its scale position remains substantially constant as shown by curve 23.

Owing to the discontinuity necessarily introduced in the heat flow path by the presence of the terminals ii, there will be a diiierence in temperature between the ends iii of the bimetal strip and the terminals I4. The greater mass and lower resistance of terminals M will tend to cause them to heat more slowly, and, consequently. to cool the adjacent ends I10! bimetal strip. As the terminals it become heated, however, owing to the flow 01' heat from the adjacent bimetallic strip, the terminals M will have less cooling eflect. This additional temperature variation in the portions ll of the bimetallic strip would introduce a supplementary bending in these por tions it they werenot channeled. This temperature variation would, consequently, cause a creep in the deflection and introduce another factor, causing a lag in the ultimate scale position of the pointer I'I. However, owing to the fact that the ends II are channeled, the portions oi the bimetallic strip, the temperature of which is eiiected by'heat absorption in the terminals ll, are prevented from bending and, since there is no discontinuity between the channel portions i3 and the remainder of the bimetallic strips i 9, the temperature along the strip in this section is substantially uniform and no creep can result.

In the arrangements of Figs. 1 and 2, I have shown bimetallic torque-producing elements in which the outer arms I! carry the current and the inner compensating arm l6 carries the deflecting pointer. It will be understood, however, that my invention is not limited to this precise arrangement. In Fig. 3, for example, the two lower arms 24 are the current-carrying arms and the upper arm 25 carries the pointer H. In either arrangement, the arm carrying the pointer I1 is heated by heat given off by the arms carrying the current.

As explained in connection with Figs. 1 and 2, the shape into which the sheet forming the bimetallic torque-producing element II is initially bent does not affect the operation of the device. If it is desired to provide a relatively flat torqueproducing element, for example, the arrangement of Fig. 4 may be employed in which the sheet forming the torque element II is bent back to have a zigzag or flattened Z-shaped section.

In accordance with the provisions of the patent statutes, I have described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof but I desire to have it understood that the apparatus shown is only 11- lustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A current-responsive instrument comprising in combination, an E-shaped sheet of bimetal, stationary supporting current connections at the ends oi! two of the arms forming said E, a pointer attached to the remaining arm of said E, and a scale cooperating therewith.

2. A current-responsive device comprising in combination, an E-shaped sheet of bimetal supported and connected at the endsof two of its arms forming the E to an electrical circuit to the current in which the device is intended to respond and means responsive to movement of the end of the said remaining arm, said remaining arm being exposed to heat dissipated from the circuitconnected arms, whereby the circuit-connected arms tend to bend due to heating in response to variations in current in said device but the end of the remaining arm tends to move oppositely with respect to the others when heated by heat dissipation irom them, whereby creepage due to gradual approach to ultimate temperature by said arms is compensated, and the time lag of response is diminished.

3. In a current-responsive device, a torque-producing element comprising a pair of bimetallic strips joined at one end to form a member having a U-shaped developed surface and supported at the mechanically unconnected ends, and terminals attached to said last mentioned ends serving to provide current connections, the ends of said bimetallic strips adjacent said terminals being channeled to prevent variation in tendency to bend which would be caused by flow of heat'irom said bimetallic strips to said terminals.

4. In a current-responsive device, a forceproducing element comprising an E,-shaped sheet of bimetal supported and connected at the ends of two of its arms forming the E to an electrical circuit to the current in which the device is intended to respond, the remaining arm being exposed to heat dissipated from the circuit-connected arms, whereby the free end of said re-' maining arm tends to move with less time lag in response to variations in current in said device than the portion of the E mechanically joining its three arms.

5. ma current-responsive device, a force-producing element comprising in combination, a sheet of bimetal having three arms joined at one end to form a member having an E-shaped developed surface and supported at the ends of two of its arms at the open portion of the E, and terminals attached to said two last mentioned ends serving to provide current connections, the ends of said arms adjacent said terminals being channeled to prevent variation in tendency to bend which would be caused by flow of heat from said bimetallic arms to said terminals.

6. In a current-responsive device, a force-producing element comprising in combination an E- shaped sheet of bimetal connected at the ends of its two outer arms forming the E to an electrical circuit to the current in which the device is intended to respond, the middle arm being exposed to heat dissipated from one of said outer arms, whereby the outer arms tend to bend in response to variations in current in said device but the middle arm also tends to bend when heated from said outer arms, moving its endoppositely with respect to the other arms, thereby-compensating creepage due to gradual approach to ultimate temperature by said arms, and diminishing the time lag of response.

'I. In a current-responsive device, a force-producing element comprising an E-shaped sheet of bimetal connected at the ends of two of the adjacent arms forming the E to an electrical circuit to thecurrentin which the device is intended to respond, the /remaining arm being exposed to heat dissipated from the circuit-connected arms, whereby the end of the remaining arm tends to move with less time lag in response to variations in current than the portion of the E mechanically joining its three arms. 1 8. A current-responsive device comprising a sheet of bimetal longitudinally slotted to form a plurality of arms joined at one end of said sheet, a stationary support to which the other end of said sheet is attached, one of said arms being free of said support and being freely suspended from the joined end of said sheet to form a compensating arm, means for subjecting the remainder of said sheet to the heating effect of an electric current, and stationary means cooperating with the end of the remaining arm to produce an eflect upon deflection thereof, the current heated portion of said sheet and said compensating arm being so related that heat dissipation from the current-heated portion affects the compensating arm and thereby reduces the time lag of response.

9. A current-responsive device comprising in combination, an E-shaped sheet of bimetal, stationary supporting current connections at the ends of two of the arms forming said E, and stationary means cooperating with the end of the remaining arm to produce an' eflect upon deflection thereof.

10. In a measuring instrument, a sheet of bimetal of E-shape, supporting current connections for the ends of two of the arms of said E, and means including a graduated scale cooperating with the remaining arm of said E for indicating the deflections thereof; said current-heated arms and said remaining arm being so related that heat radiations from the current-heated'arms 'affect the said remaining arm and thereby reduce the time lag of response.

11. A current-responsive device comprising a sheet of bimetal slotted part way from one end to form a plurality of arms with a connecting portion, a stationary support attached to the unconnected ends of a plurality of said arms, andmeans for subjecting said attached arms to the heating efiect of an electrical current,.said sheet including a portion suspended from said connecting portion to form an ambient temperature compensating arm with a free end adapted to deflect in response to variations in current.

12. In a current-responsive device a currentconduoting deflectable bimetallic strip and a slip porting terminal attached to .an end thereof to provide a current connection, the end of said bimetallic strip adjacent said terminal being channeled to prevent variation in tendency to bend which would be caused by flow of heat from said bimetallic strip to said terminal.

13. A current-responsive device comprising in' combination, a deflecting element of the bimetallic type including resistance material and adapted to deform in response to heating eflects of an electric circuit in said resistance material, a second bimetallic element, said elements being oppositely movable in response to ambient temperature changes, and a movable connection between said elements for causing algebraic addition of the movements of said elements; said elements being so related that heat dissipated from said resistance material affects said second element to compensate creepage due to gradual approach to ultimate temperature by said elements and to diminish time lag. of response to variations in current of the net motion of said elements in comparison with the time lag of response of the first element alone.

14. In a current-responsiveinstrument a deflection producing element comprising a connecting yoke and three bimetallic strips each having one end thereof joined to said yoke, supporting current connections for the unjoined ends of two of the strips, and stationa y means cooperating with the free end of the remaining strip responsive to the deflections thereof, said current heated strips and said'remaining strip being so related that ''heat radiations from the currentheated strips afi'ect the said remaining strip and thereby reduce the time lag of response.

15. A current-responsive device comprising a THEODORE A. RICH. 

